I'll share my thoughts and then others can add their opinions. Bear in mind, that driving is not my favorite mode of transport.

I'd suggest spending the last night in Switzerland in Spiez, and getting up bright and early to be on the 08:07 train, which gets them to Como just 3 hours and 38 minutes later with a change in Milano.

I'd opt to base in Como and use the ferries that traverse the lake to visit Bellagio and all the other lovely little towns and islets. I would not rent a car at all in that area.

If they think three days is to much and want a day trip, there are plenty of great places they can easily go by train (Bergamo, an hour and a half away, comes to mind).

They could either pick up their car in Como or take the train to Tuscany (either spending a couple of nights in Florence or a night in Chiusi) and pick-up their rental car there. Chiusi is also a good place to drop off the car, because Rome is just an hour and a half away by train.

I hope that helps.

Cheers! Linda